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  <title>回到顶部</title>
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  .placeholder {
    height: 130vh;
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<body>
  <div class="placeholder">
    <p>占位符，请看右下方回到顶部按钮</p>
    <p>占位符，请看右下方回到顶部按钮</p>
    <p>占位符，请看右下方回到顶部按钮</p>
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  <script src="./backtop.js"></script>
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